Hi oliverhtml
ATUM works fine on a WordPress MU but every site has its own data + config and the data can't be shared between them (at least not by default).
So if you need to separate data across distinct sites on a network and to share data, you'll need to do so with custom development. Or, alternatively, you can try to build what you want on a single site (not sure whether is an option for you though).
Best Regards,
Andrea.